At radio wavelengths,
Hook, Shaver, & McMahon
(1998),
building on the work of
Shaver et al. (1996),
have carried out an important survey that bears on the question of possible
obscuration by dust at high redshifts.
Their sample contains 442 radio sources with
S2.7GHz
0.25 Jy and stellar identifications. The highest redshift object has
z = 4.46. For objects with radio power
Plim > 7.2 × 1026 W Hz-1
sr-1, they find an evolution of the space
density very similar to WHO, SSG, and the SDSS.
This is strong evidence against dust reddening
being the main cause of the decline at high redshifts.

At the same time,
Webster et al. (1995) and
Gregg et al. (2002)
have argued that the finding of significant numbers of radio-selected
quasars with very red values of B - K in the Parkes and FIRST
surveys indicates that up to 80% of
the population is being missed in traditional optical surveys
because of dust obscuration. This interpretation has been challenged by
Benn et al. (1998) and
Whiting, Webster, &
Francis (2001)
on the grounds that
the brightness in K can arise from the emission of the host
galaxy and/or synchrotron radiation, not from obscuration
in the B band by dust. Until this issue can be resolved,
the question of dust-obscured quasars remains important to
our understanding of quasar evolution, as indicated by
the new X-ray results described below. The finding of reddened
quasars in the 2MASS survey
(Marble et al. 2003,
and references therein) is also contributing important information on
this subject.